[Bassman/NZ]

Yes you can run quite happily without thermos. A shroud will need to be made. Vinegar trick is replacing your coolant with vinegar solution and running the engine. After a day or two drain, and watch the rust pour out! The flush with a baking soda solution to neutralise the vinegar. Flush again with water, then fill with coolant. Even though you had a new radiator, old flatheads have a LOT of rust, rust flakes, casting sand and core wires in the water passages, and over time this can block up a radiator.